Hydrocarbons, such as oil and gas, may be recovered from various types of subsurface geological formations. The formations typically consist of a porous layer, such as limestone and sands, overlaid by a nonporous layer. Hydrocarbons cannot rise through the nonporous layer. Thus, the porous layer forms a reservoir, that is, a volume in which hydrocarbons accumulate. A well is drilled through the earth until the hydrocarbon bearing formation is reached. Hydrocarbons then are able to flow from the porous formation into the well.
In what is perhaps the most basic form of rotary drilling methods, a drill bit is attached to a series of pipe sections referred to as a drill string. The drill string is suspended from a derrick and rotated by a motor in the derrick. A drilling fluid or “mud” is pumped down the drill string, through the bit, and into the well bore. This fluid serves to lubricate the bit. The drilling mud also carries cuttings from the drilling process back to the surface as it travels up the well bore. As drilling progresses downward, the drill string is extended by adding more pipe sections or “joints.”
A modern oil well typically includes a number of tubes extending wholly or partially within other tubes. That is, a well is first drilled to a certain depth. Larger diameter pipes, or casings, are placed in the well and cemented in place to prevent the sides of the borehole from caving in. After the initial section has been drilled, cased, and cemented, drilling will proceed with a somewhat smaller well bore. The smaller bore is lined with somewhat smaller pipes or “liners.” The liner is suspended from the original or “host” casing by an anchor or “hanger.” A well may include a series of smaller liners, and may extend for many thousands of feet, commonly up to and over 25,000 feet.
As noted, casings are cemented in the well bore as the well is constructed. That is, the casing is smaller than the well bore in which it is installed. That gap between the casing it and the well bore is referred to as the annulus, and it is filled with cement after the casing has been installed. The cement helps to secure and reinforce the casing in the well bore and protect it against corrosion and erosion. It also supports the borehole walls from collapse. If fluids will be produced through the casing, cement helps ensure more precise control over stimulation processes, such as fracturing and acidizing. Most importantly, the cement is intended to form a complete seal around the casing. If the casing leaks, the cement will help ensure that fluids flowing through the casing do not contaminate the surrounding formation, and especially water-bearing formations. The cement also ensures that hydrocarbons and other fluids in the formation are not able to flow to the surface through the annulus.
The casing is cemented in the well bore by injecting cement, that is, a cementitious, settable composition down the casing and allowing it to flow up the annulus. Cement is basically a binder that may be formulated as an aqueous slurry which then sets, i.e., solidifies, hardens, and adheres to a material. The cement used in oil and gas wells is a hydraulic cement. Hydraulic cements are capable of setting in the presence of water. Hydraulic cement sets and adheres due to the chemical reactions between the dry ingredients (the “clinker”) and water. Portland cement, which is by far the most common hydraulic cement, is predominantly (at least two-thirds by mass) of a mixture of tricalcium silicate (3CaO.SiO2 or “C3S”) and dicalcium silicate (2CaO.SiO2 or “C2S”). The remainder of the dry components includes tricalcium aluminate (3CaO.Al2O3) or “C3A”), tetracalcium aluminoferrite (4CaO.Al2O3Fe2O3 or “C4AF”), and other minerals. The chemical reactions produce calcium silicate hydrate (CaO.2SiO2.4H2O and other mineral hydrates in various crystal phases that are essentially insoluble in water.
Portland cement was developed in the 1840s, but hydraulic cements made from volcanic ash and other pozzolana, along with lime (calcium oxide—CaO), were used by the ancient Greeks and Romans. Pozzolanas are naturally occurring pozzolans—a broad class of siliceous and siliceous-aluminons minerals which are of volcanic origin. By themselves, pozzolans have little or no cementitious properties. When mixed with lime, and in the presence of water, however, they form insoluble mineral hydrates which constitute into a cement.
Pozzolans are still used today in various cements, most commonly as a supplement to Portland cement. Calcium hydroxide in hydrated Portland cements reacts with pozzolans and is reported to form calcium silicate hydrates which can enhance the strength and quality of the resulting cement. A variety of pozzolans are known to undergo such reactions, including silica fume, metakaolin, fly ash, diatomaceous earth, calcined and uncalcined diatomite, calcined fullers earth, pozzolanic clays, calcined and uncalcined volcanic ash, bagasse ash, pumice, pumicite, rice hull ash, natural and synthetic zeolites, slag, and vitreous calcium aluminosilicate. The degree to which that reaction occurs, the different silicate hydrates formed, and the properties imparted thereby, however, are not predictable, especially given the different types of Portland cement.
Portland cements are manufactured to meet certain chemical and physical standards which in turn are reflected in standard classes and grades. Different classes and grades are suited for different applications. Cements used in oil and gas wells are subjected to wide ranges of temperatures and pressures, often in frequent and extreme cycles, which are not encountered by cements used in the construction industry and other applications. Thus, the most common standards referenced by the oil and gas industry are those promulgated by the American Petroleum Institute (API). The most common of the API classes are classes A through H, with classes G and H being the most widely used.
API cements also are graded according to sulfate resistance. The grades are ordinary (O), moderate sulfate-resistant (MSR), and high sulfate-resistant (HSR). Sulfate-resistant grades are used to prevent deterioration of a cement sheath caused by sulfates present in formation waters and other well fluids.
When cements are mixed with water, they will form a slurry, that is, a mixture of solid particles suspended in water. As a casing is cemented, the cement slurry will transform from a non-Newtonian fluid pumped into the well to a solid material filling the annular space between the casing and the borehole. Ideally, the cement will form an intimate, continuous bond with both the casing and formation, and a uniform, continuous sheath extending through the annulus without channels or voids. That is not always easy to do, however, and many different properties must be controlled and balanced to provide a strong, highly robust, low-permeability sheath.
The rheology of the slurry, such as its density, viscosity, yield strength, and thickening time are critical to the pumping phase. Those properties will determine how easily the slurry can be pumped. The slurry must be sufficiently fluid so that it can be pumped into a well, and it must remain so long enough to allow the slurry to reach the target zone which will be cemented. The slurry must not be so thick that it cannot be pumped, but it must be denser and have a higher viscosity and yield point than fluids already in the well. The slurry must displace those fluids with a minimum of mixing. Fluids mixing with the slurry can diminish the strength and quality of the cement sheath. On the other hand, the slurry must not be so heavy that it causes the formation to fracture, or forces fluid to flow into the formation, both of which can permanently damage the formation and impair production from the well.
Once the slurry is in place and pumping is stopped, the slurry must transition quickly into a solid phase and build compressive strength to prevent the flow of formation liquids and gases. Gas flowing from the formation through the slurry as it cures, for example, can create channels and leak paths in the cement sheath. It also can diminish the strength of the bond between the cement sheath and the formation.
The stability of a slurry also is highly important in creating a uniform, continuous, and impermeable cement sheath. The particulates preferably are uniformly suspended in the slurry—and remain so as the slurry hardens—so that the set cement sheath is homogeneous throughout the annulus. That will ensure that the cement has uniform properties throughout the sheath. Excess water in the slurry, that is, water added beyond what is required for the hydration reaction, tends to separate out and rise to the top of the slurry as it hardens. In a horizontal well, that “free water” or “free fluid” can create pockets or channels running along the upper part of the annulus. The channels in turn can provide paths for the flow of fluids through the sheath. Thus, the slurry should generate a very minimum amount of free water or no free water at all.
Cement typically changes volume as it cures, and those volume changes can create problems. If a cement shrinks excessively it may pull away from the casing or formation as it hardens, thus creating flow paths for fluid through the sheath. Excess expansion, however, can cause the cement to fracture, and may create harmful pressure on the casing or formation.
Once hardened, the mechanical, permeability, interfacial, hydraulic, and thermal properties of the cement sheath are critical. The cement must be strong enough to support the casing in the annulus, to maintain a continuous, impermeable sheath isolating the zone, and to withstand the mechanical and thermal shock of well operations. Moreover, the cost of drilling and completing wells is determined in large part by how long it takes to do that. Thus, the slurry also should develop strength fast enough so that the time “waiting on cement” (“WOC”) before other well operations can be started is kept to a minimum. The cement must develop sufficient strength to withstand the shock of further drilling, for example, before a new section of the well may be drilled. Even more strength may be required to perforate the well, and more still to fracture the well.
The cement sheath also must resist deterioration and fracturing over the life of the well. Fracturing can create leak paths through the sheath. In addition to its mechanical and thermal properties, its ability to resist water permeation is particularly important in maintaining the integrity of the sheath. To the extent that water can enter the cured cement, it can create micro channels in the cement that diminish the mechanical properties of the cement, thereby reducing its useful service life. Migration of water into the cement sheath is a particular concern in steam injection wells and in acidic wells. In the former, cement is simply exposed to much larger amounts of water, and especially pressurized water than is typical of most wells. In the latter, there are higher concentrations of corrosive acids that can permeate the cement.
It also will be appreciated that the economics and characteristics of a particular well may render it more suitable to a particular slurry formation. A particular formulation may provide extraordinary performance in one well and lead to complete failure in another. Cement jobs also have become more extensive. Casings have greatly increased in length over the past several years, as has the amount of cement pumped into the well. The bore hole may extend as far as 7,000 feet and may require over 600 US oil barrels (bbl) (42 gallons) of slurry to cement the casing. The increasing duration of cement jobs, especially if operations are interrupted for any reason, make it increasingly difficult to optimize cement slurries.
Fly ash has been a popular, pozzolanic additive in cements. It is known to improve various properties of cement slurries and the cured cement. Because it is produced as a byproduct of burning coal, it traditionally has been widely available from coal-fired electrical power plants at relatively low cost. The quality of fly ash, however, is not strictly maintained or controlled. It can differ significantly from batch to batch. Coal-fired plants also have come under intensifying environmental regulation. The amount of coal burned has been reduced significantly. Thus, it is increasingly difficult to obtain fly ash, its price has increased, and there is an increasing need to find substitutes for fly ash that are economical and have consistent quality.
The statements in this section are intended to provide background information related to the invention disclosed and claimed herein. Such information may or may not constitute prior art. It will be appreciated from the foregoing, however, that there remains a need for new and improved cement slurries. More particularly, there is a strong need for cement slurries incorporating pozzolans other than fly ash which have comparable or improved properties, comparable economics, and most importantly, more consistent quality. Such disadvantages and others inherent in the prior art are addressed by various aspects and embodiments of the subject invention.